Steam cracking method is most popularly used for producing lower olefins such as ethylene, propylene, butadiene and so forth from petroleum saturated hydrocarbons. About 99% of ethylene and more than 50% of propylene in the world are produced by this method. The operating conditions of steam cracking method are very stringent, for example, the maximum tube metal temperature (TMT) of the cracking furnace can reach 1125° C., and the bulk residence time of feedstocks in the radiant section tube can be 0.2 s or shorter. In the meantime, since the steam cracking products contain hydrogen, alkanes, alkenes, dienes and arenes having up to 40 or more carbons, in particular about 15 mol % of hydrogen and methane, the steam cracking products may have to be subjected to compression, complicated heat exchange, rectification and even low temperature cryogenic separation at ≦−160° C.
In view of this situation, many attempts have been made to produce lower olefins by other methods, including catalytic cracking, oxidative coupling of methane, and producing olefins from natural gas through methanol, in which the catalytic cracking methods for producing lower olefins from petroleum saturated hydrocarbons can be performed at a relatively low cracking temperature to improve the selectivity of the desired product (lower olefins) and thus catch a lot of attentions.
The methods of catalytic cracking petroleum saturated hydrocarbons may be performed in many ways including fixed bed catalytic cracking methods, fluidized bed catalytic cracking methods and so forth. Currently, the fluidized bed catalytic cracking methods (FCC technology) are primarily applied to heavy oils to generate light oils as main product and lower hydrocarbons (mainly comprising propylene) as byproducts (see, e.g., CNO2129551; CN1380898A), while the fixed bed catalytic cracking methods are mainly applied to light feedstocks such as naphtha, in which the stringency of the operating conditions for cracking petroleum saturated hydrocarbons are significantly reduced while the yields of the desired products (ethylene and propylene) are elevated. The catalytic cracking technologies that are suitable for naphtha and developed in recent years primarily pertain to fixed bed catalytic cracking technologies (see, e.g., CNO2129551; CN1380898A; CN200510028797; CNO3141148). It is believed that such fixed bed catalytic cracking reaction may increase the yield of the desired products to some extent, and may also decrease the cracking reaction temperature to some extent (relative to heat cracking reaction). However, the solid catalyst loaded in the reaction tube may cause unevenness of heat distribution in the reactor, and the coking of petroleum saturated hydrocarbons at high temperature may result in the decrease of activity or deactivation of catalyst, so that besides a component for inhibiting coking may have to be added, the amount of the dilution steam must be increased, which lead to the decrease of efficiency. In addition, the scale-up of the fixed catalytic cracking technologies may also have some problems. Investment costs for building a catalytic cracking furnace is remarkably higher than that of a steam cracking furnace with an equivalent capacity. Due to this point, the fixed bed catalytic cracking technology is still at a level far from industrialization.
Moreover, in conventional steam cracking technologies and catalytic cracking technologies, energy consumption during separation is high since the amount of small molecules such as hydrogen and methane in the cracking products is relatively great (about 15 mol %).
EP 1318187 A1 discloses an apparatus for cracking saturated hydrocarbons, in which the saturated hydrocarbons are cracked into C4-C8 unsaturated hydrocarbons, whereby propylene, butene and so forth were obtained, and in which a heat exchanger (7) can optionally comprise cracking, disproportionation and/or dehydrogenation catalysts or comprise no catalyst. That document does not give any other teachings about dehydrogenation reaction.
U.S. Pat. No. 6,586,649 B1 discloses that a product comprising 8% of ethylene, 35% of propylene and 20% of C4 fraction is obtained from a Fisher-Tropsch dehydrogenation raw material by using a C4 disproportionation technology. That document also mentions a feedstock containing butanes obtained from dehydrogenation of paraffins, but does not give any further teaching. In addition, the C4 disproportionation reaction disclosed in that document is different from the catalytic cracking reaction, and thus is not suitable for the treatment of petroleum saturated hydrocarbons, which restricts its application.
CN1317467A discloses the use of a dehydrogenation product of C4-C6 lower alkanee to improve the catalytic cracking of lower alkanes. In that process, the raw materials being treated by the catalytic cracking step are lower alkanes, in particular feed oils for catalytic cracking, which have never been dehydrogenated. The dehydrogenated lower alkanes merely act as promoters, and thus the conversion rate of their dehydrogenation is only up to 16.8 wt %. Additionally, a macroporous zeolite catalyst suitable for cracking alkanes is used in the cracking step. The Examples of that document merely relate to pure n-pentane, and from which it can be found that comparing with the situation where no dehydrogenation is performed, different conversion rates of the dehydrogenation do not significantly influence the improvement of selectivity of ethylene and propylene. For example, according to the Examples of that document, higher dehydrogenation conversion rate (e.g., 14.8 wt % of Example 6) and lower dehydrogenation conversion rate (e.g., 3.2 wt % of Example 5) result in equivalent improvement of selectivity of ethylene and propylene (e.g., the percentage is 9.89 in Example 6, and 9.26 in Example 5).
Thus, a process that uses petroleum saturated hydrocarbons as raw material is still in need, upon which energy consumption and raw material consumption are remarkably reduced, and the yield of lower olefins is significantly elevated.